Method of driving photoelectric conversion device

ABSTRACT

Disclosed is a photoelectric conversion device in which a photodiode capacitance is increased. A transparent electrode is formed between a reflecting plate and a photodiode constituting a unitary picture element of a CCD image sensor. It is so formed that light is incident from the rear surface and the loop of the standing wave of the light comes on a platinum silicide film, thereby achieving the effective absorption of the incident light. The transparent electrode is formed between the reflecting plate and the photodiode in opposition to the platinum silicide film. The capacitance between the transparent electrode and the platinum silicide film can be utilized as photodiode capacitance. Optically optimum thickness is assured by individually forming the reflecting plate which optimizes optical properties represented by the absorption of the incident light, and the transparent electrode used for increasing the photodiode capacitance, and also applying a pulse voltage to the transparent electrode at a given timing in such a manner that the potential at the time of resetting of the photodiode potential is lower than that obtained when the charge is accumulated.

This is a divisional of copending application Ser. No. 8/358,015, filedon Dec. 16, 1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a back-illuminated type photoelectricconversion device and a method of driving the same.

2. Description of the Prior Art

A conventional photoelectric conversion device and a driving methodthereof will herein be explained by way of example. FIG. 1(a) is asectional view of a unitary picture element for a CCD image sensor inwhich the conventional back-illuminated type photoelectric conversiondevices are arranged in two-dimensional configuration. In this example,an infrared sensor made of a platinum silicide/silicon Schottky diode isused as a photodiode. The photodiode is formed by bringing platinumsilicide film 1 into Schottky contact with P-type silicon substrate 2and n guard ring 3 including n⁺ region 4 is formed on the peripherythereof for suppressing leak currents. The electric charge accumulatedin the photodiode is transferred through a CCD adjacent thereto andoutputted to the outside. The CCD is composed of n well 6, transferelectrode 10 and transfer gate electrode 9.

FIG. 1(b) illustrates a driving waveform applied to transfer gateelectrode 9 for reading out the charge. A potential of high level duringa time interval of t₁ -t₂ causes transfer gate electrode 9 to be driveninto conduction and allows the potential of the photodiode to be reset,thereby concurrently effecting the reading out of the accumulatedcharge. Thereafter, the charge is further transferred by the CCD, butthe explanation on the operation thereof is omitted. The electric chargeis again read out at time t₃, and a time interval of t₃ -t₂ provides acharge storage time. This reading operation is performed every one fieldor one frame of a sensor picture plane.

In this case, the infrared ray is incident from the rear surface, sothat anti-reflection coating 12 for reducing reflection is formed on therear surface in order to make the absorption of light by the photodiodemaximum. Reflecting plate 7 is formed toward platinum silicide film 1being separated by insulating film 8 in such a manner that the loop ofthe standing wave of the light may come on platinum silicide film 1.Ordinarily, this type of reflecting plate is made of a metal such asaluminum or the like. This sensor is generally used for detectinginfrared rays having a wavelength of 3-5 μm, so that the distancebetween the platinum silicide and the reflecting plate is so adjustedthat the absorption peak occurs for a wavelength of about 4 μm.Theoretically, this optical distance is defined by an expression of(2n+1)/4 wavelength (where, n is any integer above zero), and generallychosen to be approximately 1/4 wavelength in order to make the lightabsorption at the insulating film minimum. More concretely, as shown inthe proceeding of SPIE, VOL. 1685, pages 2 through 19, the thickness ofthe insulating film is 7500 Ångstrom (hereinafter referred to as Å)inthe case of SiO₂, and 5600 Å in the case of SiO. The difference of thethickness between these two insulating films is due to the difference inthe index of refraction. Like these, the optimum value of the distancebetween the platinum silicide and the reflecting plate can be determinedbased on the selection of insulating film. In this case, the layerbetween the platinum silicide and the reflecting plate serves as part ofthe photodiode capacitance.

FIG. 4 is a diagram showing the photodiode capacitance as a function ofphotodiode reset voltage, where the capacitance is represented by thenumber of electrons which can be read into the CCD when the distancebetween the platinum silicide and the reflecting plate is varied underthe condition that the reflecting plate is grounded or not grounded. Theinsulating film used in this case is SiO₂. It can be understood from thecomparison between the results obtained when the thickness of the oxidefilm is close to the optimum value, 8000 Å (represented by squaresymbols), and the results (a curve represented by X symbols in thelowermost part) obtained when the reflecting plate is not grounded, thatthe capacitance between the platinum silicide and the reflecting platecorresponds to a large component accounting for approximately half thephotodiode capacitance. Since the background component resulting from aradiant light of 300° K. or the like is large in the infrared sensor, itis required to increase the photodiode capacitance in order to expandits dynamic range. As described above, since the capacitance between theplatinum silicide and the reflecting plate is a major component of thephotodiode capacitance, the photodiode capacitance can be effectivelyincreased by rendering the aforesaid capacitance large. However, it isknown that there exists an optimum value for the distance between theplatinum silicide and the reflecting plate in view of its opticalproperties, so that it is not possible to decrease the thickness of theinsulating film in order to increase the photodiode capacitance. Upondriving of the conventional photoelectric conversion device shown inFIG. 1(a), the grounding of the reflecting plate and the application ofpulse voltage are not performed unlike the present invention. When thereflecting plate is not grounded, the capacitance between the platinumsilicide and the reflecting plate is not available because the potentialin the reflecting plate becomes equal to that of the photodiode.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide aback-illuminated type photoelectric conversion device capable of makingthe photodiode capacitance large, while accomplishing optimum opticalproperties for enabling the absorption of incident light maximum, and amethod of driving the same.

According to the first present invention, the back-illuminated typephotoelectric conversion device comprises a reflecting plate disposed,via an insulating film, in opposition to a photodiode formed on asemiconductor substrate of the photoelectric conversion device forreflecting light incident from the substrate side through the photodiodeand the insulating film toward the photodiode side, wherein thereflecting plate is grounded.

In the preferred embodiment of the aforesaid first invention, theoptical distance between the photodiode and the reflecting plate isequal to (2n+1)/4 (wherein, n is any integer above zero) of thewavelength of the incident light wavelength.

According to the second present invention, the back-illuminated typephotoelectric conversion device comprises a reflecting plate disposed,via an insulating film, in opposition to a photodiode formed on asemiconductor substrate of the photoelectric conversion device forreflecting the light incident from the substrate side through thephotodiode and the insulating film toward the photodiode side, and atransparent electrode disposed opposite to the photodiode in between thereflecting plate and the photodiode and also transparent to the incidentlight, wherein the transparent electrode is grounded.

In the preferred embodiment of the aforesaid second invention, thereflecting plate may be either grounded or not, and the optical distancebetween the reflecting plate and the photodiode is equal to (2n+1)/4(wherein, n is any integer above zero) of the incident light wavelength.

According to the driving method for the back-illuminated typephotoelectric conversion device corresponding to the first presentinvention, a pulse voltage is applied, at a given timing, to thereflecting plate in such a manner that the potential of the reflectingplate becomes lower at the time of resetting of photodiode potentialthan that obtained at a time when the photodiode charge is accumulated,and according to the driving method for the back-illuminated typephotoelectric conversion device corresponding to the second presentinvention, a pulse voltage is applied, at a given timing, to thetransparent plate in such a manner that the potential of the transparentplate becomes lower at the time of resetting of photodiode potentialthan that obtained at a time when the photodiode is discharged.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a sectional view of a unitary picture element in aconventional two-dimensional CCD image sensor.

FIG. 1(b) is a timing chart showing a driving voltage waveform appliedto a transfer gate electrode upon operation of the element of FIG. 1(a).

FIG. 2 is a sectional view illustrating the construction of oneembodiment of the photoelectric conversion device according to thepresent invention.

FIG. 3 is a timing chart showing one embodiment of the driving voltagewaveform used for driving the photoelectric conversion device accordingto the present invention.

FIG. 4 is a diagram showing photodiode capacitance as a function ofphotodiode reset voltages, wherein the photodiode capacitance isrepresented by the number of electrons which are read into a CCD whenthe distance between the platinum silicide and the reflecting plate isvaried under the condition that the reflecting plate is grounded or not.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Herein, will be provided an explanation of preferred embodiments of thepresent invention, by reference to the drawings attached.

The first embodiment of the photoelectric conversion device of thepresent invention has the same construction as that of the prior artexample in FIG. 1(a), except that reflecting plate 7 is grounded. Asalready explained in connection with FIG. 4, the capacitance of thephotodiode in the case where the thickness of the insulating filmlocated between the platinum silicide and the reflecting plate isassumed to be 8000 Å is increased almost double as compared to thecapacitance of conventional photodiode in which reflecting plate 7 isnot grounded.

Referring now to FIG. 2 corresponding to the second embodiment of thepresent invention and illustrating a sectional view of a unitary pictureelement for reading out the charges accumulated in the photoelectricconversion device by the CCD in the same way as that already mentionedin conjunction with FIG. 1(a), transparent electrode 13 is disposedwithin the insulating film sandwiched between platinum silicide film 1and reflecting plate 7, unlike the conventional construction of FIG.1(a). As transparent electrode 13, a metallic thin film such as theplatinum silicide or the like having a thickness of less than 100 Å, asemiconductor with a band gap larger than the energy of the wavelengthto be photoelectrically converted, and a conductive metal oxide such asITO, SnO₂, TiO₂, etc., can be used. In the case of the infraredphotoelectric conversion device aiming a wavelength of 3-5 μm range, itsenergy is less than 0.4 eV, so that a semiconductor such as polycrystalsilicon, amorphous silicon, and ZnSe, etc., having a band gap largerthan that range can be used as transparent electrode 13. In this case,transparent electrode 13 has some degree of thickness in view of itsresistance value. However, since the optimum optical distance betweenthe platinum silicide and the reflecting plate lies in approximately(2n+1)/4 wavelength (where, n is any integer above zero) as described byreferring to the conventional example, the optimum optical propertiescan be accomplished by adjusting the value of integer n. The reflectingplate can be either grounded or ungrounded. Also, the platinum silicidethin film can be used as transparent electrode since it exhibits atransmittivity of 80% or more for a wavelength of 3-5 μm if thethickness thereof is on the order of 40 Å. In this case, the thicknessof the transparent electrode must be thin adequately and a wavelength ofabout 1/4 is used as the optical distance.

As the capacitance between the platinum silicide and the transparentelectrode is defined by the capacitance of a parallel-plates capacitorwith a dielectric between the plates, the smaller the gap between theplates, the larger the capacitance becomes, as can be seen from FIG. 4.As compared with the conventional device of FIG. 1(a), the aforesaid gapis reduced from the gap between the platinum silicide and the reflectingplate to the gap between the platinum silicide and the transparentelectrode, so that the photodiode capacitance is increased. In short,according to the photoelectric conversion device of the presentinvention, an increase in the photodiode capacitance can be realizedwhile accomplishing the optimum optical characteristics making theabsorption of incident light maximum.

The foregoing discussion was performed as to the specific case where theCCD was used as a reading circuit. However, because the presentinvention lies in the improvement of characteristics of thephotoelectric conversion unit, it also applicable to any sensorutilizing other reading circuits such as MOS and CSD (Charge SweepDevice) or the like. In addition, it can be applied to either of asingle sensor and a one- or two-dimensional array sensor.

Herein, an embodiment of driving method for the photoelectric conversiondevice of the present invention will be explained in detail. As thephotoelectric conversion device in this case, either the structure whereonly the reflecting plate is provided as in the aforesaid firstembodiment or the structure where the transparent electrode isadditionally provided as in the aforesaid second embodiment can beapplied.

Referring to FIG. 3 corresponding to one embodiment of the drivingmethod according to the present invention, and illustrating a timing ofpulse voltage applied to the electrode (either of the reflecting plateor transparent electrode) located opposite to the transfer gateelectrode and the platinum silicide, the pulse voltage is applied to theelectrode opposite to the platinum silicide, unlike the conventionalexample shown in FIG. 1(b). As can be seen from the waveform diagram, ahigh level of voltage is applied to the transfer gate electrode 9 duringa time interval of t₁ -t₃, so that the potential of the photodiode isreset to a reset voltage V_(rst), thereby reading the charge into theCCD. The voltage of the opposed electrode is switched from V_(A1).str toV_(A1).rst at the time T₂ at which the photodiode is being reset andthen returned to V_(A1).str at the time t₄ after the reset period.

After the potential of the photodiode is reset to V_(rst), the chargewill be accumulated until the potential becomes 0V since it falls as theaccumulation of charge proceeds. Assuming now that the number ofelectrons accumulated in the photodiode at the potential V_(PD) is N(V_(PD)), N (V_(PD) =0) cannot be read out, so that the effective amountof charge, N_(PD), read into the CCD will be expressed by the followingformula:

    N.sub.PD =N(V.sub.PD =V.sub.rst)-N(V.sub.PD =0)

Since the number of electrons accumulated within the capacitance betweenthe platinum silicide and its opposed electrode, which is part ofN_(PD), is given by ε_(di) (V_(PD) -V_(A1))S/q d_(di), the effectiveamount of charge, N_(di), obtained in the case where such pulse voltageas shown in FIG. 3 is applied is given by the following formula:

    N.sub.di =ε.sub.di (V.sub.rst -V.sub.A1.rst)S/q d.sub.di -ε.sub.di (0-V.sub.A1.str)S/q d.sub.di =ε.sub.di V.sub.rst S/q d.sub.di +ε.sub.di (V.sub.A1.str -V.sub.A1.rst) S/q d.sub.di

where, ε_(di) and d_(di) show the dielectric strength and the thicknessof the insulating body, respectively, V_(A1) is the voltage of theopposed electrode, S shows the area of diode, and q shows an elementarycharge. In the event that the potential of the opposed electrode isconstant, the second term becomes 0 since V_(A1).str =V_(A1).rst,whereas, if V_(A1).str >V_(A1).rst, the photodiode capacitance isincreased by the second term.

Although the first and second embodiments have been explained in thecase where the transfer gate is controlled by the transfer gateelectrode, the present invention is also applicable to the case wheretransfer electrode 10 is extended up to the region of the transfer gateand a ternary pulse is applied to transfer electrode 10.

As heretofore explained, according to the photoelectric conversiondevice or the method of driving the same based on the present invention,a remarkable increase in the photodiode capacitance may be attainedwhile accomplishing the optimum optical properties that can make theabsorption of incident light maximum.

Although certain preferred embodiments of the present invention havebeen shown and described in detail, it should be understood that variouschanges and modifications may be made therein without departing from thescope of the appended claims.

What is claimed is:
 1. A method of driving the back-illuminated typephotoelectric conversion device of the type comprising a reflectingplate electrode disposed, via an insulating film, in opposition to aphotodiode formed on a semiconductor substrate of the photoelectricconversion device for reflecting light incident from the substrate sidethrough said photodiode and said insulating film toward the photodiodeside, wherein said reflecting plate electrode is grounded, said methodcomprising the step of applying a pulse voltage to said reflecting plateelectrode at a given timing in such a manner that the potential of saidreflecting electrode at the time of resetting of the photodiodepotential is lower than that obtained at the time when the photodiodecharge is accumulated.
 2. A method of driving the back-illuminated typephotoelectric conversion device of the type comprising a reflectingplate disposed, via an insulating film, in opposition to a photodiodeformed on a semiconductor substrate of the photoelectric conversiondevice for reflecting light incident from the substrate side throughsaid photodiode and said insulating film toward the photodiode side, anda transparent electrode disposed between said reflecting plate and saidphotodiode in opposition to said photodiode and transparent to theincident light, wherein said transparent electrode is grounded, saidmethod comprising the step of applying a pulse voltage to saidtransparent electrode at a given timing in such a manner that thepotential of said transparent electrode at the time of resetting of thephotodiode potential is lower than that obtained at the time when thephotodiode charge is accumulated.